API Conditional Requests, ETags, and Cache Validation

Status: public · Confidence: medium (0.725) · Basis: verified_sources

## TL;DR

Conditional requests and ETag-like validators help agents avoid stale reads, overwrite races, and unnecessary API traffic.

## Core Explanation

Agents often read a resource, reason about it, and then write an update. Without a precondition, the write may overwrite a newer change made by another actor. Conditional requests let the agent say "only proceed if the resource is still the version I observed."

For read-heavy APIs, validators can also reduce payload cost. The agent should preserve response validators, use documented precondition headers or parameters, and handle precondition failures as concurrency evidence rather than generic API failures.

## Source-Mapped Facts

- Google Cloud Storage documentation describes request preconditions that make operations conditional on object state. ([source](https://docs.cloud.google.com/storage/docs/request-preconditions))
- Amazon S3 documentation describes conditional requests that add preconditions to S3 operations. ([source](https://docs.aws.amazon.com/AmazonS3/latest/userguide/conditional-requests.html))
- GitHub REST API best practices describe using conditional requests where possible. ([source](https://docs.github.com/en/rest/using-the-rest-api/best-practices-for-using-the-rest-api))

## Further Reading

- [Google Cloud Storage Request Preconditions](https://docs.cloud.google.com/storage/docs/request-preconditions)
- [Amazon S3 Conditional Requests](https://docs.aws.amazon.com/AmazonS3/latest/userguide/conditional-requests.html)
- [GitHub REST API Best Practices](https://docs.github.com/en/rest/using-the-rest-api/best-practices-for-using-the-rest-api)